the 1500 crewcabs only come with a 5'8" or 6'6" bed. Gas tank is 26 gallons with either. You are going to find that most dealers will not have many with the 6'6" as that makes the truck to BIG for some people, remember it has to fit in the garage. It really depends on what you are taking with you. i have had both just adapted what I was taking with. When you go 2500hd you have the option of 6'6" or 8' again finding the 8' is not easy. The fuel tank is 36 gallons with either box. Find ones you like and drive see which you like best